It's a good setup, but I too think you need gravel.
Also, are you going for a natural look? If you are, I suggest getting driftwood to replace the shells, b/c shells (at least for me) invoke images of a marine tank.
Just remember, slope the gravel backward (so the height is lowest towards the front) and place tall plants in the back and small up front. Leave the space in the middle open for swimming and move the net breeder to the side.
These are all opinions (and some facts) and I'm sorry if you already knew this stuff.